Field of the Invention
The present invention relates to a wide angle lens, and an image pickup apparatus using the wide angle lens, and particularly to a wide angle lens of an interchangeable lens camera, and an image pickup apparatus using the wide angle lens of the interchangeable lens camera.
Description of the Related Art
As an interchangeable lens camera, a mirrorless single lens camera has been known. In the mirrorless single lens camera, there is no bending mirror as in a single lens reflex camera. Therefore, it is possible to make the mirrorless single lens camera more compact as compared to the single lens reflex camera. In recent years, the compactness of the mirrorless single lens camera has been well received by users, and a market for the mirrorless single lens camera has been expanding.
The mirrorless single lens camera, similarly as the single lens reflex camera, is provided with an AF (auto focus) function. Here, as a type of AF, there is a phase-difference AF and a contrast AF. The phase-difference AF has been predominant in single lens reflex cameras.
On the other hand, in the mirrorless single lens cameras, there are cameras in which, the phase-difference AF cannot be used. In such mirrorless single lens cameras, the contrast AF is to be used. In the contrast AF, focusing is carried out by finding a location at which, the contrast is the maximum, by scanning a focusing lens unit.
A weight of the focusing lens unit poses a problem in a case of using the contrast AF. Here, an amount of movement of the focusing lens unit which is necessary till a focused state is assumed, is let to be a predetermined amount of movement. In a case of the phase-difference AF, the predetermined amount of movement can be calculated at a time by using information from an AF sensor. Therefore, it is possible to move the focusing lens unit according to the predetermined amount of movement that has been calculated.
On the other hand, in a case of the contrast AF, information acquired from the AF sensor is a contrast value at that instant. In other words, it is not possible to calculate the predetermined amount of movement at a time. Therefore, in the contrast AF, the contrast is calculated upon moving the focusing lens unit, and is compared with the contrast before moving. In such manner, the focusing is to be carried out while finding a location at which, the contrast is the maximum, while reading a change in the contrast.
Here, for detecting the maximum value of the contrast, it is to be confirmed that the contrast after the movement becomes smaller than the contrast before the movement. Therefore, in the contrast AF, it is necessary to move the focusing lens unit further beyond a position at which the contrast became the maximum.
Consequently, in a case in which, the predetermined amount of movement is compared for the contrast AF and the phase-difference AF, the predetermined amount is predominantly larger for the former. From the aforementioned points, in an optical system in which the contrast AF is to be used, light-weighting of the focusing lens unit becomes a major issue.
As a wide angle lens in which, the contrast AF is used, optical systems disclosed in Japanese Patent Application Laid-open Publication Nos. 2013-257395, 2013-238740, 2012-173435, 2012-226309, 2013-218267, and 2013-037080 are available.